The invention relates to an apparatus for measuring internal dimensions or corrosion of a pipeline.
The steel wall of a pipeline may be subject to internal corrosion depending, obviously, on the matter that is transported through the pipeline. Both liquids and gases may attack the steel wall and produce two forms of corrosion; namely, uniform corrosion and pitting. In the case of uniform corrosion, the wall thickness decreases more or less uniformly, and it will be clear that this will eventually lead to the pipeline becoming unserviceable, because the wall will become too weak to resist the internal pressure. Pitting leads to leakage, and this phenomenon must also be avoided. It is, obviously, of great importance to constantly know the state of the pipeline to be able to take corrective action in time.
Since downtime is costly, it is preferred that the inspection of pipelines be carried out while they are in operation. Moreover, pipelines are often installed where access is difficult, e.g. underground or underwater, so that methods of internal inspection are preferred.
U.S. Pat. No. 2,896,332 discloses an apparatus for inspecting the interior of a pipeline comprising a pig carried along by the flow of matter through the pipeline. The pig is fitted with a circular array of spring-loaded probes protruding radially outwards, and pressed against the inner wall of the pipeline. The probes are fitted with displacement pickups, and the pig is equipped with apparatus for processing the signals from the displacement pickups and recording the results of such processing. The pig is further fitted with means for the determination of the distance traveled by the pig. This may be a time clock whose signal is regularly recorded and may be used in conjunction with the pumping schedule to determine the location of the pig at any time. It is also possible to use a supporting wheel whose revolutions indicate the location of the pig.